1. Field of the Invention
This invention relates to a lithium ion secondary battery in which positive and negative electrodes face each other by sandwiching a separator which keeps electrolytes, more particularly to a thin lithium ion secondary battery having excellent charge and discharge characteristics.
2. Description of the Related Art
There is a growing demand for the miniaturization and lightening of portable electronic instruments. In order to realize such a demand, it is necessary to improve performance of batteries, such as large voltage, large energy density and tolerance for large load resistance, as well as optional shaping, secure safety and the like, so that development and improvement of various types of batteries are now in progress. Lithium ion battery is a secondary battery which can realize the highest voltage, largest energy density and tolerance for largest load factor resistance among existing batteries and is expected as a most probable candidate which will satisfy the aforementioned requirements.
As its main composing elements, the lithium ion secondary battery has a positive electrode, a negative electrode and an ion conducting layer inserted between these positive and negative electrodes. In the lithium ion secondary batteries which are now put into practical use, a plate-shaped material prepared by mixing powder of a lithium-cobalt oxide or the like active material with a binder resin together with powder of an electron conducting substance and coating the mixture on an aluminum collector is used as the positive electrode, and another plate-shaped material prepared by mixing powder of a carbonaceous active material with a binder resin and coating the mixture on a copper collector is used as the negative electrode. Also, a porous film such as of polyethylene, polypropylene or the like filled with a lithium ion-containing non-aqueous solution is used as the ion conducting layer.
In the currently available lithium ion secondary batteries, electric connection between positive electrode-ion conducting layer-negative electrode is maintained by pressurizing the system using a strong armor case made of stainless steel or the like. However, the just described armor case increases weight of the lithium ion secondary battery and causes difficulties not only in miniaturizing and lightening the battery but also in making it in optional shapes due to rigidity of the armor case.
In order to miniaturize and lighten the lithium ion secondary battery and to obtain its optional shapes, it is necessary to bond a positive electrode to an ion conducting layer, and a negative electrode to the ion conducting layer, and maintain such conditions without applying external pressure.
An example of such a method has been disclosed in U.S. Pat. No. 5,437,692 in which a lithium ion conducting polymer is used in an ion conducting layer, and a positive electrode and a negative electrode are bonded to the ion conducting layer using an adhesive layer containing a lithium compound. Also, a method in which a thermoplastic ion conducting layer is formed and positive and negative electrodes are bonded together using the thermoplastic ion conducting layer has been disclosed in WO 95/15,589.
However, by the aforementioned method disclosed in U.S. Pat. No. 5,437,692, sufficient bonding strength cannot be obtained and sufficiently thin shape as a battery cannot be obtained, and it also has a practical problem in terms of charge and discharge characteristics and the like battery characteristics due to large ionic conduction resistance between the ion conducting layer and positive and negative electrodes. Also, the aforementioned method of WO 95/15,589 has problems in that sufficient bonding strength cannot be obtained because of the use of a thermoplastic ion conducting layer in the bonding and sufficiently thin shape cannot be formed as a battery.